Wednesday's episode of CSImarks the CBS procedural's 300th episode, and no matter how you slice it, that's a lot of murder."We figure we've killed thousands of people," executive producer Carol Mendelsohn says with a laugh. "Which probably isn't something to be proud of, but... most shows never get to 300 episodes. Everybody's just so proud."The CSI cast reflects on the show's finest hoursIndeed, reaching such a milestone is rare in today's increasingly fragmented television landscape. CSI's success is particularly remarkable when you consider that the show's Miami- and New York-based spin-offs have both come and gone while the original trucks on. "When we first started to spin off Miami and then CSI: NY, I figured the last one standing would still be the mothership," Mendelsohn says. "It was just our instincts. It had nothing to do with talent, quality, storytelling. It's just that the original CSI so defined what a procedural was. It opened up all of us - writers, audience, everyone - to this world of forensics, which I'd never heard of before. For 14 seasons, the audience has been learning along with us... and I think that's probably why the fans have remained loyal to the original show."Realizing the show would reach the milestone this season, executive producer Don McGill says they wanted this entire season in general, and the 300th episode in particular, to look back at the show's history. "When Carol and I talked about the season, [we thought,] 'Back to the beginning,'" McGill says. "And looking at episode 300, literally we do go back to the beginning. We reference a case that was the one that got away for our CSIs."Marg Helgenberger returning to CSI for 300th episodeThe case hits particularly close to home for Sara (Jorja Fox). "It was her first high-profile case when she came to Las Vegas," Fox tells us. "She's getting a second chance at something. She's been dealing with the family for all these years and not having the answers. There's a weight and a sadness that she carries. But by the end, I think she absolutely gets some closure. And she feels fairly anchored, as much as Sara can."The case involves a reclusive casino mogul (guest star Jason Priestley) who is suspected in a crime eerily similar to the case from 14 years ago. As the CSIs dig for clues, the show uses what McGill calls "faux-backs" to tie the two cases together. In addition to seeing Sara and Greg (Eric Szmanda) in circa 2000 looks, the episode also welcomes back original cast member Marg Helgenberger as Catherine Willows."We created these little vignettes," McGill says. "It gave us a chance to have a window in the evolution of the characters. You could really see how far Sara has come, how far Greg has come, the influence of Catherine. It was a really fun opportunity to give a nod to how great the show is and how its longevity has really created this dynamic where we've seen the characters grow and evolve."Exclusive: CSI books Lea Thompson for November sweeps

Page 2 of 2 - The episode will feature lots of Easter eggs, including 14 different references to the number 300. But Mendelsohn says the episode also pays appropriate homage to the show's history. "Everybody's been asking, 'What are your favorite episodes? What moments do you remember the best on set?'" she says. "And because we want to hark back to those iconic moments of the past, we flash back to some of the golden oldie cases. We took a poll among all the writers: What are your favorite cases? And those are the cases we flash back to during the episode."In particular, the show pays homage to the show's original protagonist and leader, William Petersen's Gil Grissom. Although Grissom has only appeared a handful of times since the character was written out in Season 9, his spirit seems to always loom over the action. Mendelsohn gives Petersen credit for much of the show's success.

"Billy said, 'I do not want Gil Grissom to always be right.' That became our mantra," Mendelsohn says. "It allowed us to create this incredible, iconic, forensic detective. Because of Billy's instructions, he was both a student of the world and a teacher of the world. He has had such longevity and such a continuing presence on the show [because] Grissom taught all of our CSIs. And he was the teacher of all of us.

"[He took] us into a world that none of us had any exposure to and then to make science entertaining and exciting," she continues. "Grissom held our hand and took us on that journey to the truth. So, we are reverential and referential to Grissom, but it's because Gil Grissom started us all on the path to forensics. He was a new kind of detective. And in a world that was often gray, Grissom always led us to the truth."And will that legacy lead to another 300 episodes? "Greg says something to Sara at the end of 300," McGill says. "It's something to the effect of, 'We're only just getting started' I think Carol and I agree."CSI airs Wednesdays at 10/9c on CBS.